Superconducting metal-ceramic laminate

ABSTRACT

This invention permits superconducting ceramics, as well as other ceramic materials, to be spray deposited onto indefinitely large sheets of metallic substrate from a carboxylic acid salt solution. Elemental metal precursors of the superconductor are introduced into the solution as carboxylic acid salts. The deposit formed on the malleable metallic substrate is then thermomechanically calcined to form c-axis textured metal-superconductor composite sheet structures. These composite sheet structures can be formed by pressing together two ceramic-substrate structures, ceramic face-to-face, to form a metal-ceramic-metal sheet structure, or by overlaying a metal sheet over the deposited structure. Once the structure has been thermomechanically calcined, the c-axis of the superconductor is oriented parallel to the vector defining the plane of the metal sheet, i.e., perpendicular to the surface of the plane. These sheets of c-axis textured superconductor can then be mechanically worked as continuous superconducting filaments into larger composite structures with predetermined c-axis orientation. The fact that the c-axis of the superconductor is given predetermined orientation allows surface topologies to be mechanically constructed from these sheets that maximize the performance of the superconductor for a predetermined application or magnetic field environment.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

RELATED APPLICATIONS

This is a divisional of 08/733,867 filed Oct. 18, 1996, now U.S. Pat.No. 5,866,252 which is Continuation of 08/441,905, filed May 16, 1995,which is a divisional of 08/263,207 filed Jun. 16, 1994, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates generally to metal composite wire or sheetstructures containing low-temperature (low-T_(c)) or high-temperature(high-T_(c)) superconducting (HTSc) ceramic oxides. In these structures,the superconductor is deposited as a deposit, within a metal compositematrix with or without additional metallic, metal alloy, or ceramicfilamentary components. The present invention relates, in particular, tothe design, process fabrication and construction of HTSc-metal compositewires or sheets of finite or indefinite continuous length, in which theHTSc ceramic is embedded within or deposited on a metallic sheet that isused in the final composite structure. The superconducting ceramic isused either as a single filament (superconducting layer), or as multiplefilaments, or as either in association with other metallic or ceramicfilamentary components that provide greater mechanical, thermal orelectrical stability to the function of the composite structure.Furthermore, each filamentary component containing HTSc ceramic isimparted a predetermined c-axis orientation relative to the longitudinalor radial axis of the wire, or planar axis of the sheet. Application ofthis c-axis textured ceramic sheet allows the designer to improve theelectromagnetic performance characteristics of the composite structure.Superconductors conduct electromagnetic power without resistive losswhen cooled below an intrinsic thermodynamic transition temperature,commonly defined as the T_(c) of the material. In addition to allowingthe resistanceless transport of electromagnetic power, thesuperconductive state also nulls electric fields and expells magneticflux (lines of force) from within the interior of the material. Thiscombination of properties allows superconductors to be useful in avariety of electromagnetic systems that require electromagnetic powerstorage, power delivery, power regulation, low loss power transmission,power amplification, or electromagnetic shielding.

Electromagnetic power can be stored within a superconducting wire orsheet when it is wound or formed into a topological surfacerepresentative of a magnetic coil or solenoid. Power is supplied to thesuperconductor by driving an electrical current through it from anexternal source. This generates an electrical current through it from anexternal source. This generates an electrical supercurrent within thesuperconductor. This is known as charging the superconductor. When thesuperconductor is sufficiently charged, a superconducting short circuitis activated between the ends of the coil structure, so the electricalsupercurrents circulating within the coil may follow a closed loopcontinuous superconductive path. Since the superconducting path does notdissipate the electrical power, the supercurrents persist indefinitely.After the superconducting storage device as been charged, the storedpower can be tapped to supply power to an electrical grid.

This same configuration can be used to regulate power to the grid whenactive electrical switches and relays that monitor the grid areconfigured to tap the electrical power in the superconductor when thegrid is experiencing an electrical energy surplus. Since thesuperconductor is an extremely low loss electrical power conduit,cables, bus bars, or leads, can also be used to transmit power from anelectrical energy source to the load with negligible power loss, orpower loss that is significantly less than that of normally resistivepower feeds. Since the superconductor has no dc electrical resistivity,it can be used to transfer electrical power in a cryogenic environmentwith negligible heat generation.

Radio frequency (rf) electrical power is amplified by a cavityresonator. The physical dimensions and dielectric constants of thematerials used to construct the cavity determine the frequencies thatwill maximally experience power amplification within the resonator.Quality-factors (Q-factors) characterize the gain per unit frequencywithin the resonator structure. Q-factors can be greatly enhanced,allowing significant improvements to rf gain over a narrower band of rffrequencies if the walls of the cavity structure are coated with a layerof superconducting material.

Since the superconducting state thermodynamically prevents electric andmagnetic fields from penetrating its interior, a hollow superconductingsurface enclosing field sensitive instrumentation can shield thesedevices from harmful external electromagnetic radiations. Likewise, if amagnetic field source or electromagnetic radiation source is placedwithin a hollow superconducting shell, the superconducting surface canbe used to confine or constrain field emissions or to shape magneticfield emissions protruding out a hole in the superconducting shell.

These embodiments of superconducting topologies are useful in a varietyof electrical systems that require the efficient utilization of theavailable power budget, such as an airborne or space-based system. Theyare also useful in devices that require the efficient manipulation,generation, or delivery of powerful electromagnetic pulses, forinstances in electromagnetic weaponry or electromagnetic rail guns; or,the regulation, storage and transmission of electric power over anelectrical grid; or, the amplification of an rf power source, forinstance in electric countermeasure devices or radar systems; or toprotect sensitive electronic equipment against electroniccountermeasures or interfering radiation.

All of these applications require the superconducting material to becooled to a temperature below its thermodynamic transition temperature.If the material is heated above its T_(c) it will fail to operate as asuperconductor and revert back to its state of normal resistance,causing the unique functionality of the material to be lost to theapplication. The application of magnetic fields and electrical currentsto the superconductor can also stress the thermodynamic state ofsuperconductivity. If the superconductor is maintained at the lowestpossible temperature and no electrical currents are passing through it,a threshold magnetic field can be applied above which magnetic flux isno longer expelled from the material and it fails to remainsuperconducting. The value of magnetic field that causes thesuperconducting state to rupture is defined as the critical magneticfield (H_(c)). If the temperature of the superconductor is increased toa value that is elevated but still below its T_(c), the intensity of anapplied magnetic field that will rupture the superconducting state isless than the H_(c) measured at the lowest possible temperature. Theintensity of magnetic field that is needed to rupture the superconductoris an increasing function of decreasing temperature below T_(c) of thesuperconductor. This relationship can also be interpreted as meaning theT_(c) of the superconductor is a decreasing function of increasingapplied magnetic field intensity.

The physical representation of this relationship can be mapped onto agraph using temperature and magnetic field intensity as axes, with aline defining the boundary between superconducting and normallyresistive states of the material. This line is referred to as theirreversibility line of the superconductor. All values of field andtemperature that are within this line (closer to the origin of the axes)allow the material to retain its superconductive properties. All valuesof field and temperature that are exterior to this line (further fromthe origin of the axes) rupture the superconductive state of thematerial.

A similar functionality is observed with electrical current travelingthrough the superconductor. At the lowest possible temperature, and inthe absence of an applied or generated magnetic field intensity, thesuperconductor will be able to transport a maximal level of electricalcurrent density. If the current density is increased beyond this maximallevel, known as the critical current density (J_(c)) of thesuperconductor, the superconductive state is ruptured. As is the casewith applied magnetic field intensity, the J_(c) decreases withincreasing temperature below T_(c). It can alternatively be stated thatthe T_(c) of the superconductor decreases with increasing current load.

Superconductivity was initially discovered in certain pure metals (suchas mercury, lead, vanadium, niobium and tin) that are cooled to very lowtemperatures, generally less than 4 K. These superconductors, known astype-I superconductors, expell all magnetic flux from their bulkinteriors until a critical magnetic field intensity is applied. Thecritical magnetic field intensities, critical current densities, andcritical temperatures of these superconducting pure metals are so lowthat utilizing the phenomenon of superconductivity with them has limitedpractical value.

It was subsequently discovered that certain impure metals and metallicalloys, like niobium-tin or niobium-germanium, also exhibitsuperconductivity at elevated temperatures and higher magnetic fieldintensities. What distinguishes these superconductors from the type-Isuperconductors is that the superconducting state still persists eventhough some of the applied magnetic flux actually penetrates theirinterior without rupturing the superconducting state. The penetratingflux is confined to tubular clusters known as fluxoids. In normallyresistive material all of the lines of applied magnetic flux areuniformly distributed throughout the material. In this mixed state of asuperconductor the penetrating flux lines are packed into discretetubular flux clusters. Equal groupings of flux lines are channeled intothe fluxoids, causing the flux line density to increase within thefluxoid and be zero between fluxoids. Inside the volume of the fluxoidthe superconductor is in a state of normal electrical resistance, whileoutside the microscopic volume of the fluxoid it retains itssuperconducting properties.

Under stable conditions of thermodynamic equilibrium, these bundles ofmagnetic flux distribute themselves with uniform cluster densities overthe superconducting surface and maintain their equilibrium positions. Alarger fraction of the superconductor's volume remains superconductingif fewer fluxoids penetrate the superconductor. Fluxoid penetrationincreases with increasing superconductor temperature below T_(c).Superconductors that exhibit this "mixed state" are known as type-IIsuperconductors. These materials have greater practical use since theyretain their superconducting properties at higher temperatures thantype-I superconductors, and at magnetic field intensities common to manypractical applications.

Superconductivity has also been observed in certain ceramic materials,such as barium-potassium bismuthate (Ba_(x) K_(1-x) BiO₃) and a varietyof copper-oxide ceramics, including yttrium-barium-copper oxide (YBa₂CU₃ 0), and specific material phases of certain bismuth cuprate(Bi--Pb--Sr--Ca--Cu--O), thallium cuprate (Tl--Ba--Ca--Cu--O), andmercury cuprate (Hg--Ba--Ca--Cu--O), ceramics. The cupratesuperconductors exhibit type-II superconductivity at significantlyhigher superconducting transition temperatures (90-140 K), and arereferred to as high-T_(c) superconductors. The bismuthate ceramic is anisotropic low-T_(c) superconductor.

It is thus defined that the superconducting state is a function of thematerial, its temperature, the current density flowing through it, andthe magnetic field intensity applied to it. In order for asuperconductor to be applied in any of the applications mentioned above,the environment in which it is operating must be designed to maintainthermodynamic conditions that prevent the material from transitioningfrom the superconducting state into its state of normal electricalresistance. The ability to maintain control over the thermodynamic stateof the superconductor can be improved by enveloping the superconductoras a single, or as multiple, filamentary strands(s) within a metalcomposite matrix. The specific metals or metallic alloys, as well astheir relative physical dimension, used in this composite structure areselected on the basis of their intrinsic physical properties and theability of these properties to relieve or mitigate the occurrence andpropagation of energetic disturbances or instabilities that developwithin the superconductor as it is operated.

Energetic disturbances that can compromise the performance of activesuperconductors are known to have either a mechanical origin or be theresult of magnetic flux jumping. Mechanical disturbances could be due tothe gradual or catastrophic release of physical stresses that develop asa result of electromagnetic loading on the mechanical component(s) ofthe superconductor, or due to transient or steady state vibrations thatdevelop within the superconductor as electrical current passes throughit. Magnetic flux jumping describes the sudden and dissipativerearrangement of magnetic flux within a superconductor. It ispredominantly generated by the repulsive electromagnetic interaction ofthe penetrating lines of magnetic flux with electrical currentstransported through the superconductor. Magnetic flux orientedperpendicular to the path of moving electrical charge is subject to anelectromagnetic force, known as the Lorentz force, F_(L).

In isotropic superconductors, the magnitude of the Lorentz force on asingle bundle of penetrating magnetic flux is:

    F.sub.L =θΦ.sub.o sin ⊖.                 (1)

where θ is the current flow per unit area passing by the flux bundle, Φis the magnetic flux density contained within the bundle, and ⊖ is theangle subtended between the direction of the current flow and theorientation of the magnetic flux bundle. The effect of this force on theelectrical current causes it to be deflected from its original path. Theeffect of this force on stable penetrating flux bundles causes them tobe dislodged from their equilibrium positions. The acquired kineticenergy of the moving fluxoids is eventually dissipated within thesuperconductor as heat.

Often these energetic disturbances occur at specific points within thesuperconducting structure. If the energetic point disturbance issufficiently long lived, the accumulated thermal energy dissipated bythe disturbance may be sufficient to locally trigger the superconductingstate to revert back to its state of normal resistance at that point.Current passing through this normally resistive point will startgenerating greater quantities of heat, which, if not quickly transportedto a thermal reservoir, can precipitate catastrophic failure along theentire length of the conductor. An objective for embedding thesuperconductor within a metal composite structure is to providethermally conductive pathways that drain the dissipated heat at ratesfaster than the energetic disturbance can generate it in thesuperconductor. Superconducting metal composite structures are alsouseful because the metal provides a low resistivity electrical pathwayto shunt current in the superconductor if it goes normal. Filamentarycomponents can also be selected to improve the mechanical integrity ofthe composite through strength membering, or to dampen unwantedvibrational modes, thus relieving the incidence of mechanical energeticdisturbances.

The selection of metallic components interfaced with the superconductormust also be compatible with its material processing requirements. Thesintering kinetics of high-T_(c) superconducting ceramics require thatthe ceramic precursor be hermetically encased in a silver sheath if theyare to be efficiently processed into the more desirable superconductingphases. Silver has very high rates of oxygen diffusivity at elevatedtemperatures. Exposing the ceramic to controlled oxygen atmospheres isneeded to regulate the thermal reaction kinetics while processing theprecursors into the desired phase of the sintered ceramic. Somecomponents of high-T_(c) superconducting ceramics can become volatile inoxygen partial pressure atmospheres that are favorable to the reactionwhen elevated to the necessary process temperatures. The hermetic silverbarrier blocks evaporative or liquid loss of the more unstable precursorcomponents while still permitting use of optimal oxygen atmospheres tofavorably regulate the chemical kinetics of the sintering reaction.

Composite wire structures of high-T_(c) superconducting ceramics arecurrently manufactured by billet processing techniques. In billetprocessing, a stoichiometric blend of precursor powder, referred to asthe billet, is packed into a silver tube which is hermetically sealed atboth ends. The billet can either be a blend of elemental metallicprecursors, elemental metal oxide precursors, or powders of distinctmicrostructural phases of the ceramic that are known to react favorablyor efficiently into the desired single phase of the ceramic duringsubsequent processing. Depending upon which billet process is used, thistubular composite structure can then be mechanically deformed andelongated into a monocore wire and reacted to form the superconductingphase, or pre-reacted and drawn into monocore wire structures before thefinal thermal sintering of the superconductor.

Existing billet-processing techniques allow precursor powders to bedeformed into monocore (single filament) wires. Multifilamentary wirestructures produced using this technique are fabricated by fusingmultiple monocore powder-in-tube structures into a large bundle, anddrawing the entire composite through a die to reduce its overalldiameter and to extend its length before reacting the formed wire. Inboth cases, the superconductor is in intimate contact and fullyenveloped by the silver casing.

Often it is desirable to have metals other than silver in intimatecontact with the superconductor to reinforce its thermal or mechanicalperformance. The selection of what metallic composition would beusefully applied in intimate contact with the superconductor dependsupon whether or not it would be more useful to improve the thermal ormechanical performance of the superconductor in the application forwhich the composite structure is intended. Since precursors powders arenecessarily packed into a sealed tube, billet processing techniques onlysilver, or metals with comparable oxygen diffusivities at temperaturesused to process the superconductor, to have intimate contact with thesuperconductor. The composite structures made using these metals andthis technique may not allow the superconductor to achieve optimalperformance standards within the composite.

Billet processing techniques have other practical drawbacks. While theycan be used to synthesize long lengths of wire, it is not a processingtechnology that is well suited for many of the application listed above.For instance, high-T_(c) superconducting electrical power transmissionbetween a power station and a substation will require extremely longcontinuous lengths of superconductor. Billet processing only permits thefabrication of composite structures of finite length determined by theinitial dimensions of the tube into which the billet is packed and theextent to which it is deformed. Other possible applications, such aselectromagnetic shielding or superconducting rf cavity resonators mayrequire large continuous sheets of high-T_(c) superconducting ceramic,or exposed ceramic surfaces that cannot be made using billet processing.

Another approach to improving the performance of the superconductingcomposite structure is to implement design structures that reduce theincidence of magnetic flux jumping in the superconductor. Flux jumpingcan be hindered by introducing microscopic defects that act as fluxpinning centers. Fluxoid bundles are immobilized at these centers, andconsequently will not dissipate kinetic energy into the superconductorprovided the flux pinning potential of the center is greater than theenergies the fluxoids are experiencing through Lorentz forceinteractions with currents in the superconductor. In multiple elementsuperconductors, i.e., not pure metals, flux jumping can be reduced byfabricating the superconductor using a materials synthesis process thatachieves fine subdivision of the individual precursors. Powder synthesistechniques do not allow extremely fine precursor subdivision.

A more fundamental problem to billet processing is its inability toimplement innovative composite design architectures that reduce magneticflux jumping in the high-T_(c) superconducting ceramics. When the billetis packed into the silver tube, the crystallographic orientation of theprecursor powders are randomly oriented with respect to one another. Thefinal ceramic reaction product is c-axis textured by thermomechanicallysinter processing--(heating and flattening)--the entire compositestructure. Consequently, as a result of this thermomechanical sinteringstep, the finished phase in the ceramic filament(s) is produced with itsc-axis oriented uniquely in one direction.

A fundamental property of the high-T_(c) superconducting ceramics istheir crystallographic anisotropy. The crystallographic structure ofthese ceramics is characterized by the crystallographic c-axis, the longaxis of its crystalline unit cell, which is perpendicular to the basalor a-b-crystallographic plane. Superconductivity in the high-T_(c)cuprates is exhibited primarily along the basal plane of the ceramic.Superconductive phenomena, as measured by critical current densities andcritical field intensities, is considerably weaker in these ceramicsalong orientations parallel to the crystallographic c-axis. This isparticularly true at elevated temperature and in applied magneticfields. Consequently, these ceramics are more susceptible to fluxjumping in magnetic fields that are oriented parallel to the ceramicc-axis than they are to magnetic fields oriented along their basalplane.

This anisotropy is less exagerated at low temperatures, i.e., near 4 K,but becomes increasingly and dramatically pronounced at temperaturesabove 10-15 K. Therefore, despite the high superconducting (onset)transition temperatures for these ceramics (90-127 K), this fundamentalproperty can severely limit their practical application unlesssuperconducting surfaces and multifilamentary composites can beconstructed with predetermined c-axis topologies that favorably orientthe ceramic to the anticipated magnetic field lines of a givenapplication. Depending upon the application, this will require someceramic filaments in the structure to have their c-axis oriented indirections that are different from others. This cannot be achieved usingbillet processed composites in which c-axis texturing is achieved, if itall, during the final sintering step, usually causing all of the ceramicfilaments to have similar c-axis orientation if they arethermomechanically processed.

Thus, there exist a need for a more effective manufacturing process toconstruct high-T_(c) superconducting multifilamentary compositestructures. The improved manufacturing process should allowsuperconducting ceramic components to have predetermined c-axisorientation within the composite structure, as well as provide a meansto place the superconducting components in intimate contact withfilamentary metals or metal alloys other than silver without sacrificingan oxygen diffusion pathway to the exterior of the composite structureduring atmosphere-controlled thermal processing. This manufacturingprocess should also allow other ceramic filamentary components, whichhave electrically resistive or magnetorestrictive properties that arefavorable to the stable function of the composite, to be incorporatedwith similar ease. Finally, this manufacturing process should also allowcomposite structures to be fabricated to indefinite continuous lengthsor surface areas.

SUMMARY OF THE INVENTION

The present invention provides a manufacturing process to construct amultifilamentary composite structure using superconducting ceramic, orother ceramic components, that have been imparted predetermined c-axisorientation, and the ability to manufacture such structures withindefinite continuous length or surface area. This invention isparticularly useful to the manufacture of composite structurescontaining high-T_(c) superconducting ceramic filamentary components,which are known to have poorer resistance to magnetic flux jumping ifapplied or generated magnetic fields are oriented parallel to theircrystallographic c-axis, particularly at temperatures above 10-15 K.

This capability can allow the construction of high-T_(c) superconductingmetal composite architectures designed to function at higher operatingtemperatures than what is currently capable in composite structuresconstructed using more conventional synthesis methods. It further allowsspecific architectural designs to be implemented for specificsuperconducting applications.

This process uses a technique that easily achieves ultrafine subdivisionof the precursor elements that are reacted to form the superconductingmaterial, thereby improving the magnetic flux jumping characteristics towhich it can be made.

Therefore, one object of the present invention is the method used tofabricate (deposit and construct) a superconducting metal compositestructure in which the superconducting filament(s) have been impartedpre-determined c-axis orientation.

Another object of the invention is to implement innovative designarchitectures that favorably orient the c-axis of the superconductingfilament(s) to the anticipated magnetic field lines of the applicationin which the composite structure is intended to be used.

Another object of the invention is the utilization of the process toimplement innovative design architectures that incorporate c-axistextured ceramic filament(s) with magnetostrictive, electricallyinsulating, or piezoelectric properties to stabilize the function of thesuperconducting filaments in the composite.

Another object of the invention is its ability to easily add to thecomposite other metallic or metal alloy filamentary components inintimate contact with the superconducting filaments to improve thethermal or mechanical characteristics of the composite structure.

Another object of the invention is the method to manufacture complexceramic materials, that is, materials that have three or more elementalprecursor components, to higher quality than can be achieved usingbillet or powder processing techniques.

These and many other objects and advantages of the present inventionwill be readily apparent to one skilled in the pertinent art from thefollowing detailed description of a preferred embodiment of theinvention and the related drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Sheet of c-axis textured ceramic metal composite.

FIG. 2. Illustrates the irreversibility lines for the 2212 phase of thebismuth cuprate, the 2212 and 1223 phases of the thallium cuprate, andthe 123 phase of the yttrium-barium-copper oxide families of high-T_(c)superconducting ceramics.

FIG. 3. Illustrates the crystallographic anisotropy in the 2223 bismuthcuprate high-T_(c) superconducting is shown by measuring the criticalcurrent density in the material with magnetic fields appliedperpendicular (⊥) and parallel (∥) to the crystallographic c-axis of theceramic.

FIG. 4. Magnetic field pattern induced by current in a dc transmissionline.

FIG. 5. Preferred embodiment of a transmission line with an HTSC groundplane.

FIG. 6. Magnetic field pattern of a solenoid in the half plane.

FIG. 7. Magnetic field pattern overlaying the c-axis orientation of amonocore HTSc ceramic solenoid in the half plane.

FIG. 8. Preferred embodiment of an HTSC solenoid with c-axis texturedfilaments oriented to be perpendicular to the magnetic field pattern.

FIG. 9. Thermogravimetric analysis is used to show thermal decompositionspectra of carboxylic acid precursor salts in a solution used tomanufacture bismuth cuprate HTSc ceramic.

FIG. 10. XRD spectra displaying improvement in the ceramic c-axistexture in an Ag/Bi₁.68 Pb₀.31 Sr₂.20 Ca₂.17 Cu₃.6 /Ag composite after(a) a 6 hour, 830° C. oxygen calcination treatment and one interveningmechanical cold press; and, (b) a calcination treatment totaling 15hours with two intervening mechanical cold presses.

FIG. 11. A metal/ceramic/metal composite structure incorporating silveron one face and a stabilizing metal on the other face.

FIG. 12. Spliced joint in a metal/ceramic/metal composite structure.

FIG. 13. Utilizing splice joint technology to form a closed topologicalsurface from a single composite sheet.

FIG. 14. Assembling metal ceramic composite sheets into a larger andmore complex composite structure.

FIG. 15. Use of fine Ag particles to form a hermetic seal at asilver-silver interface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention permits ceramic superconducting materials to be spraydeposited onto indefinitely large sheets of metallic substrate from acarboxylic acid salt solution. Elemental metal precursors of thesuperconductor are introduced into the solution as carboxylic acidsalts. The deposit 14 formed on the malleable metallic substrate 10,FIG. 1, is then thermomechanically calcined to form c-axis texturedmetal-superconductor composite sheet structure 12. These composite sheetstructures 12 can be formed by pressing together two ceramic-metalsubstrate composite structures, 10 and 14, ceramic face-to-face, to forma metal-ceramic-metal sheet structure, or by overlaying another metalsheet over the deposited structure 10, 14 and 10. Once the structure 12has been thermomechanically calcined, the c-axis 16 of the principalmicrostructural phases of the superconductor is oriented parallel to thevector defining the plane of the metal sheet, i.e., perpendicular to thesurface of the plane. These sheets of c-axis textured superconductor canthen be cut to prespecified dimension and mechanically worked ascontinuous superconducting filaments 18, FIG. 14, into larger compositestructures with predetermined c-axis orientation. The ceramicfilament(s) with the composite will maintain this c-axis orientationafter it (they) is (are) sintered into the final reaction product, i.e.,the single ceramic phase ultimately desired in the composite.

Superconductors are compositional states of matter that lose allelectrical resistance when they are cooled to temperatures below anintrinsic thermodynamic transition temperature, commonly defined as theT_(c) of the material. In addition to allowing the resistancelesstransport of electromagnetic power, the superconductive state alsolimits the penetration of electric and magnetic fields into their bulkinteriors. This phenomenology makes them extremely useful for a varietyof technological applications.

Central to the technological application of these materials is therequirement to maintain the superconducting material in itssuperconductive state. Superconducting materials exhibit this phenomenonwithin well defined regions of temperature and magnetic field, which isrelated to fundamental intrinsic characteristics of the material. FIG. 2shows the irreversibility line for distinct phases of variouscompositional families of high-T_(c) superconducting ceramics. Iftemperatures and magnetic field intensities beyond those thresholdvalues are applied to these superconductors, their superconducting stateis irreversibly lost. In this instance the magnetic field must becompletely removed and the superconductor must be allowed to cool againbefore superconductivity can once again be observed.

The density of electrical current passing through the superconductor canalso cause the superconductor to revert back to its state of normalresistance if it attains a value beyond a critical threshold. Thecritical current densities which most superconductors can sustain areusually related to extrinsic characteristics of the materials, such asthe way they were processed.

For superconductivity to be technological useful it must be embodiedwithin an environment that allows it to sustain the superconductingstate within physical parameters defined by its temperature, theelectrical current density passing through it, and the intensity of anapplied or self-generated magnetic field. If any region of thesuperconductor is exposed to temperatures, magnetic field or currentdensities beyond these critical thresholds, energetic disturbances willbe generated within that region of the superconductor. These energeticdisturbances are thermally dissipated in the superconductor, causing itslocal temperature to rise. If the heat generated by the disturbance isnot drained quickly enough, the region will revert back to its state ofnormal resistance. Current passing through this normally resistiveregion will start to generate greater heat through resistive heating,which, if not quickly transported away from the superconductor can causecatastrophic failure along the entire length of superconductor.

The energetic disturbances that can trigger catastrophic failure havetwo root causes. They may originate from the release of mechanicalenergy that builds up as a result of electromagnetic forces acting uponmechanical components, or be the result of magnetic flux jumping withinthe superconductor.

A preferred embodiment of superconductors is to introduce them asfilamentary components within a metal composite structure. Thecomposition of the metallic matrices surrounding the superconductor isselected to have intrinsic properties, such as higher heat capacitiesand thermal conductivities than the superconductor, that allow heatgenerated within the superconductor to be quickly transported to acooling reservoir without significantly increasing the temperature ofthe composite. The composite structure may also contain elements thatact to improve the mechanical integrity of the composite under theeffect of electromagnetic forces without compromising the ability tothermally process the superconductor.

Another means to improve the performance of the superconductor is byreducing flux jumping within it. That is achieved most directly throughmaterials processing, either by improving the subdivision of theprecursor materials used to form the superconductor, or by introducingmicroscopic flux pinning centers into the superconductor.

High-T_(c) superconductors are not as successfully applied toconventional embodiments due to the crystallographic anisotropy in theirsuperconducting properties. FIG. 3 demonstrates crystallographicanisotropy in the 110 K phase of the bismuth cuprate compositionalfamily. The superconductive state is more easily sustained if magneticfields are oriented perpendicular to the c-axis of the ceramic. Thisstate is much more easily ruptured by flux jumping when the field isoriented parallel to the ceramic c-axis, particularly at temperaturesgreater than 10-15 K.

Therefore, it is the preferred embodiment of this invention to constructcomposite structures from high-T_(c) superconducting ceramics that haveincreased resistance to flux jumping energetic disturbances by orientingthe c-axis of high-T_(c) superconducting filamentary components indirections perpendicular to the magnetic field lines anticipated fromthe technological application.

FIG. 4 shows a transmission line 20 with a current passing through itscore 22. The passing current induces a magnetic field 24 that isperpendicular to the direction of the current flow and loop around thewire 20. Given the orientation of magnetic fields induced by thedirection of current flowing in the conductor, it is now possible to usethis invention to construct a composite structure, FIG. 5, useful as atransmission line 26 containing a high-T_(c) superconducting groundplane Such a composite structure would have a superconductingtransmission line core 28, surrounded by concentric filamentarycomponents, at least one of which acts as an electrical insulator 30. Anouter filamentary component 32 acts as the ground plane and containshlgh-T_(c) superconducting ceramic with its c-axis oriented in theradial directions and perpendicular to the magnetic field linesgenerated by currents flowing in the core 28 of the transmission line26. This structure is constructed by cutting a width of c-axis texturedmetal-superconducting-composite sheet 36, FIG. 13, to match the radialcircumference of the transmission line and mechanically wrapping thesheet as a continuous cylindrical casing 34 around the transmissionline. This method of construction orients the c-axis of the ceramic inthe radial direction for the full 360° of arc about the longitudinalaxis of the transmission line. The ground plane c-axis is, therefore,oriented tangentially to the fields generated by longitudinal currentsin the transmission line core.

It is straightforward to demonstrate how this technique can be appliedto composite structure deployed in more complicated field environments.FIG. 6 shows the lines of magnetic field 28 generated by a singlesolenoid loop in a cross-sectional half-plane of the solenoid. The fieldis generated by current running along the conductor with rectangularcross-sectional areas i e., into the page in FIG. 6. As shown, themagnetic field pattern of this solenoid configuration can cause intenseradial field components to be located within the conductor. If thissolenoid were constructed from a billet-processed high-T_(c)superconducting wire with a c-axis textured monocore the composite wouldbe extremely susceptible to energetic disturbances that would develop inthose regions where the radial field components are oriented parallel tothe c-axis of the ceramic FIG. 7. Unsuitable magnetic field orientationswould cause these anisotropic components to fail except at temperatureswhere all the magnetic flux is expelled or the anisotropy is lifted.

Manufactured sheets of c-axis textured ceramic filament can be cut tospecified width and assembled into a larger metal composite structurewith other metallic or metal alloy filaments to construct a structuresimilar to that shown in FIG. 8. Composite structures 40 with c-axistextured high-T_(c) superconducting filaments organized in patternssimilar to that shown in FIG. 8 are useful because they providesuperconducting current paths without significantly exposing the weakaxis of the superconductor to the radial fields of the solenoid. Aseries of looped current paths are formed to generate the magnetic fieldbut the composite structure will experience less flux jumping since thec-axes of the individual filaments have more favorable alignments withthe magnetic field patterns. Design s similar to this can improvehigh-T_(c) superconductor performance at temperatures above 15 K.

There are additional benefits to this process. It is unique as a wire orsheet manufacturing process because it rapidly achieves ultrafinesubdivision of the individual precursors by mixing them as metalorganicacid salts in an organic acid solution. Prolonged grinding, milling, andreaction processing of precursor powders are not required in thisprocess. Mixing the elemental components in solution allows them toachieve subdivision at the molecular level in a matter of minutes. Thissolution is then spray deposited onto a suitable hot metallic substrate.The heat supplied by the substrate causes the salts to pyrolyze into anoxide deposit at the substrate surface, forming the initialmetal-ceramic precursor composite structure.

The use of spray deposition is unique because it preserves the ultrafineprecursor subdivision better than any other solution applicationtechnique. Each of the dissolved precursor salts pyrolyze into anirreducible oxide form at different decomposition temperatures. When adissolved salt decomposes into its oxide form it precipitates from thesolution. Depending upon the rate at which the solution is heated,differences in the decomposition temperatures of the presursor salts cancause the microstructure of decomposed metal oxides to phase separatethrough their sequential decomposition. For instance, assume a solutionis prepared to contain two salt precursors and one of the saltsdecomposes at a temperature of 265° C., while the other decomposes above500° C. As the solution is heated from room temperature to a temperatureabove 500° C., the salt that pyrolyzes at 265° C. will decompose,precipitate from solution, and separate, at the molecular level, fromthe salt that decomposes at 500° C. When the solution reaches atemperature above the 500° C., the second salt will decompose, leaving asolution precipitate that is largely composed of two distinctmicrostructural phases.

Sequential decomposition can be reduced when processing ceramics fromsolution by engineering solutions that contain precursor salts thatdecompose over a very narrow temperature range, by polymerizing all thedifferent metallic precursors into a molecular structure that decomposesat a single temperature, or heating the solution at a rate that issufficiently rapid to achieve nearly simultaneous decomposition of allthe salts. The synthesis of a precursor salt solution from a carboxylicacid-based chemistry allows all of the precursor salts to decompose overa fairly narrow temperature, and is one of the preferred embodiment ofthis invention. FIG. 9 uses a thermogravimetric trace to characterizethe narrow (ΔT=100° C.) range of decomposition temperatures forcarboxylic acid precursor salts useful in synthesizing high-T_(c)superconducting ceramic in the Bi--Pb--Sr--Ca--Cu--O compositionalfamily.

The use of an aerosol spray decomposition technique is a requisite andpreferred embodiment of this invention because it achieves far greaterrates of heating than can be achieved by applying the solution to thesubstrate by spin-coating or dip-coating techniques, and thus maintainsthe ultrafine precursor subdivision achieved in solution. Even if theprecursor salt solution is engineered to have a narrow decompositionrange, the rates of heating that can be achieved by simply placing aprecursor film that was coated onto a room temperature substrate in ahot furnace is still not sufficient to prevent sequential decompositionand separation of the precursor oxide phases. An aerosol spray alsocauses all of the precursor salts to be contained within the volume of adroplet. Hence, if sequential decomposition does occur, precursor phaseseparation is constricted by the droplet size, allowing precursors to besubdivided at least within a physical proximity defined by the averagedroplet size.

The application of the precursor salts using an aerosol spray alsoallows composite structures of indefinite continuous length or surfacearea to be formed. Since this spray application technique can bepreformed in an open chamber, there is no physical limitation to thesurface dimension of metallic substrate that can be fed into the systeminput. Manufacture of extremely long continuous lengths can be achievedby winding the metallic substrate on to a payout spool and feeding itinto the system off the spool. Once deposited, the composite structurecan be collected onto a pick-up spool and stored for further processing.

The use of solutions prepared from carboxylic acid salts dissolved intoan organic solvent is a requisite and preferred embodiment of thisinvention. This chemistry allows very high solution concentrations, upto ≈25% equivalent weight oxides, to be engineered. Contiguous ceramicdeposits cannot be formed easily on metallic substrates via spraypyrolysis without this solution chemistry or high solutionconcentrations. Spray pyrolysis of HTSc ceramic precursors usingchemistries other than the one defined causes a coating of looselyconnected powder to form on the substrate that can be easily blown offits surface.

The method of solution preparation is also an embodiment of theinvention. Carboxylic acids are organic compounds that contain thecarboxyl group (--COOH) ##STR1## where R is either an alkyl or an arylgroup. These acids are easily converted into non-volatile salts that canbe held together in a crystalline solid by strong electrostatic forces.The temperatures that are required for melting the lattice of acarboxylic acid salt is so high, usually 300-400° C., that carbon-carbonbonds break and the molecule decomposes before the melting temperatureis reached. This combination of low volatility and low-temperaturepyrolysis within a relatively predictable range of decompositiontemperatures makes these salts ideal for the spray decomposition ofceramic deposits.

These solutions can be prepared by mixing assayed proportions of saltsin organic solution, or by sequentially reacting elemental precursormetals or acetate precursor salts with a carboxylic acid solution.Sequential reaction appears to reduce the formation of alkaline earthcarbonate phases in sprayed deposits. Alkaline earth precursor metalsare formed into salt precursors through the following reaction:

    M.sup.o +2▪RCOOH==>M.sup.2+ (OOCR).sub.2 +H.sub.2 (g)(M=Ca, Sr, Ba),                                                      (2)

All other metal precursor components can be formed into salts byintroducing them to the carboxylic acid as acetate salts, then drivingan exchange reaction and distilling off the acetic acid, i.e.,

    M(OOCH.sub.3).sub.X +X▪RCOOH==>M.sup.X+ (OOCR).sub.X +H.sub.3 COOH.                                                     (3)

The spray deposited ceramic formulation generally has a spongy, poroussolid solution microstructure, which requires further processing beforeit can be brought into a physical condition that makes it useful to thisinvention. A finely subdivided, homogeneous blend of oxide precursorsrequires thermal processing in controlled atmospheres. The thermalprocessing can be broken down into two distinct procedural steps,calcination and sintering. The initial deposit is calcined to decomposeany residual organic material that may not have decomposed during spraypyrolysis. As the oxide precursor deposit is heat treated, recognizablemicrostructural phases of the ceramic begin to nucleate and grow.Certain distributions of these microstructional phases favor oraccelerate the conversion of the ceramic into a single high-T_(c)superconducting phase during sintering. Failure to influence the propermicrostructional phase distribution during calcination generallycomprises the ceramic's superconducting properties after sintering.Therefore, a secondary objective of the calcination process is totransform the deposit from an oxide solid solution into a specificmultiple phase ceramic. After calcination, the ceramic should have amicrostructional phase chemistry that can be rapidly and reliablyconverted into the single desired HTSc phase during sintering.

This principle is most easily explained using the layered HTSc ceramicsas an example. These compositional families are composed of oxides ofone heavy-metal element--(bismuth and lead (Bi,Pb), thallium (TI,) ormercury (Hg)), two alkaline earth components (calcium (Ca), strontium(Sr,) or barium (Ba)) and copper (Cu). The compositional familiesinclude (Bi, Pb)--Sr--Ca--Cu--O, Tl--Ba--Ca--Cu--O, andHg--Ba--Ca--Cu--O. Each of these compositional families have similarcrystalline phase chemistries. They have 20-30 K superconducting phaseswith 2201 phase chemistries, i.e., (Bi,Pb)₂ Sr₂ Ca₀ Cu₁ O₆, anintermediate temperature (75-95 K) superconducting phase designated asthe 2212 phase, i.e., (Bi,Pb)₂ Sr₂ Ca₁ Cu₂ O₈, and a high-T_(c) (>105 K)superconducting phase with 2223 phase chemistry, i.e., (Bi,Pb)₂ Sr₂ Ca₂Cu₃ O₁₀. Each of these phase are similar, except a layer of CaCuO₂ hasbeen intercalated into the crystalline structure of the progressivelyhigher-T_(c) phase.

The most effective reaction pathway to synthesize the 2223 phases duringsintering is to react calcined microstructional phase distribution of2212 ceramic with cuprous oxide (CuO) and calcium cuprate (Ca₂ CuO₃) inthe following strict proportions:

    2212+0.5 Ca.sub.2 CuO.sub.3 +0.5CuO==>2223                 (4)

Typical conversion of the 2212+secondary phases in silver sheathedcomposites consists of an induction stage followed by the conversionreaction, which can best be described by a two-dimensionalnucleation-growth mechanism. The 2223 phase adopts the c-axisorientation of the 2212 ceramic phase prior to the sinter reaction.

It is therefore a preferred embodiment of this invention tothermomechanically calcine the sprayed deposit by interleavingheat-treatments with mechanical presses. This action has the effect ofincreasing the rate at which a sprayed deposit with 2223 stoichiometryis converted from an ultrafinely subdivided homogeneous solid solutionof precursor oxides into 2212 ceramic+secondary phases. It further hasthe effect of texturing the 2212 phase in the ceramic with c-axisorientation predominantly along the vector mathematically describing theplane of the substrate, i.e., perpendicular to its surface. As shown inFIG. 10, the repeated short heat treatments with intervening mechanicalpresses creates the c-axis textured metal-ceramic composite sheet (36)structure that is central to the invention.

This stage of the process may be performed using a composite structureconsisting of a single ceramic layer on metal with its surface exposed,or by pressing together two such structures, ceramic face-to-face, toform a sandwiched metal-ceramic-metal composite structure. Thesandwiched structures would not be useful in applications that requirean exposed superconducting surface, such as in a superconducting rfcavity resonator. However, these sandwich structures have enormousutility in the design and construction of high-T_(c) superconductingmetal composite structures, where it may be advantageous to theperformance of the composite to have a metal other than silver inintimate contact with the superconductor.

It is therefore a preferred embodiment of the invention to constructmetal-ceramic-metal sheet structures, FIG. 1, in which the ceramic 14,between the metal layers 10 has its c-axis textured by thermomechanicalcalcination. It is, additionally, a preferred embodiment of theinvention to construct these sheet structures by pressing together twodifferent ceramic metal composite structures, one which has the ceramic42 deposited on silver 10, or a metal 44 with comparable oxygendiffusivities at HTSc processing temperatures, to another, which has itsceramic deposited directly on a metal with intrinsic physicalproperties, such as a Young's modulus, heat capacity or thermalconductivity, that stabilizes the performance of the superconductor inthe composite. (See FIG. 11). When structures such as this are embeddedwithin a silver composite matrix, it is then possible to mechanically orthermally stabilize the ceramic filament with the metal on one of itssides, and still provide an oxygen diffusion pathway between the ceramicand the composite's exterior through the silver on the other side of thefilament.

The transition temperatures of high-T_(c) superconductors increase whena compressive force is applied to the ceramic. This allows higherthermodynamic loads, currents and magnetic field intensities, to beapplied to these ceramics when they are in a state of compression. Thisinvention can be used to manufacture composite structures using ceramicswith magnetostrictive properties. Magnetostrictive materials changetheir crystallographic dimensions when magnetized. The amount ofdimensional change observed is a function of the ceramic composition,the applied magnetic field strength and its relative orientation to thecrystallographic c-axis of the ceramic. Some materials aremagnetostrictive along the c-axis, while others exhibit this property inthe a-b plane. As they expand (or contract) in one crystallographicorientation, they contract (or expand) in the perpendicular plane.Composites containing c-axis textured filaments with these propertiescan be used to apply compressive loads to superconducting filaments, ifsuitably configured. Therefore an embodiment of this invention includesits use to manufacture ceramic filamentary sheets that can directionallyalter compressive stresses on components in a larger composite structurein a manner that is proportional to the applied or generated magneticfield intensity.

Another embodiment of this invention, FIG. 12, allows smaller compositesheet structures to be spliced into larger sheets without disrupting thecontinuity of the superconducting layer, or to use a splice joint 46 toform a closed ceramic or superconducting surface topology from asingle-sheet of composite. Spliced joints with a continuouslyinterconnected superconducting layer are easily formed using thesandwiched metal/ceramic/metal composite structures. This is achieved byallowing one of the metal-ceramic sheets pressed into the sandwich tohave an edge 48 that hangs over the side of the other. This overhangingedge can be mated with an underhanging edge 50 from another sandwichstructure and mechanically pressed together.

FIG. 13 shows how a continuous closed ceramic topological surface, inthis case a cylinder 34, can be constructed from a single compositesheet. The sheet sandwich structure is constructed using twoceramic-metal composite sheets of equal surface dimensions. The topsheet is laterally displaced from the bottom sheet along one direction,causing it to overhang on one side and to underhang on the other. Thesheets are mechanically pressed and wrapped into a cylinder where theoverhanging edge mates with the underhanging edge. An additional pressalong the seam closes the ceramic loop.

Sintering high-T_(c) superconducting ceramics from a homogeneousdistribution of microstructional phases into a pure single phase withthe highest-T_(c) requires the use of controlled oxygen atmospheres,which can simultaneously decompose the (thermomechanically) calcinedceramic if it is not hermetically sealed in a silver sheath. Silverallows the oxygen atmosphere exterior to the composite to regulate theconversion reaction (equation (4)) and prevents loss of the morevolatile components of ceramic. A principal advantage to this inventionis that these c-axis textured ceramic sheets can be cut to specifieddimension and assembled into a larger composite structure as filamentsand have the same c-axis orientation in the assembly after sintering.This is not possible using billet processing techniques in which thecalcined powders are packed into a tube with random c-axis orientationsprior to sintering.

A method for embedding c-axis textured sheets into a larger silvercomposite, FIG. 14 uses a sheet of silver sheathing and mechanicallydeforms it into well shaped structure 52. The c-axis textured sheets canthen be mechanically shaped (if necessary) and inserted into the wellwith other metal or ceramic filamentary components (if desired). Theplanar orientation of the sheet placed in the well determines the c-axisorientation of the filamentary component. Metallic or ceramic compositespacer filaments can be added to the composite to adjust relativefilament spacing(s) to conform to the design intended for the composite.Side walls to the well can then be folded over the assembly and closethe composite at a silver-silver seam. (See FIG. 14). This process stepis compatible with the manufacture of continuous composites ofindefinite length since each of the filaments can be drawn from a payoutspool and be spliced into longer lengths before being placed in thesilver casing. (See FIG. 15). Fine silver particles are preferred sinceother metallic particles could form an alloy with the silver sheathingthat has a eutectic melting point at temperatures required to sinter theceramic embedded within the composite. These fine particles can beapplied to the surfaces of the sheathing material using a silver loadedepoxy, paste, slurry, or electroless plating compound. They can beadhered to the surface of the silver by either curing and firing theepoxy or paste, or by drying the slurry. The surface activity of thesefine silver particles is enhanced by their small dimension. Once theseam has been mechanically pressed, these fine particles can berecrystallized at relatively low temperatures to hermetically seal theseam.

What is claimed is:
 1. A metal-ceramic composite comprising, a laminatewhich has:a) a pair of spaced metal layers, b) a superconducting (sc)layer of c-axis textured metal-ceramic oxide positioned between saidmetal layers and in contact therewith and c) said c-axis being orientedin one direction.
 2. The composite of claim 1 wherein said c-axis isoriented at an angle relative to magnetic field lines applied to orgenerated by said composite.
 3. The composite of claim 2 wherein saidc-axis is oriented substantially perpendicularly to said magnetic fieldlines.
 4. The composite of claim 1 wherein said laminate is rolled intoa superconductive tube around an sc core to define an outer concentricsc ground plane of a coaxial cable.